On the origin of cold-dense plasmas in the dusk magnetotail plasma
sheet: MMS Observations
Abstract
The near-Earth plasma sheet becomes cold and dense under the northward
interplanetary magnetic field (IMF) condition, which suggests entry of
solar wind plasma into the magnetosphere across the magnetopause. The
cold and dense characteristics of the plasma sheet are more evident in
the magnetotail flank regions that are interface between cold solar wind
plasma and hot magnetospheric plasma. Several physical mechanisms have
been proposed to explain the entry of solar wind plasma across the
magnetopause and resultant formation of the cold-dense plasma sheet
(CDPS) in the tail flank regions. However, transport path of the
cold-dense plasma inside the magnetotail has not been understood yet.
Here we present a case study of the CDPS in the dusk magnetotail by
Magnetospheric Multiscale (MMS) spacecraft under the conditions of the
strongly northward IMF and the high-density solar wind. The ion
distribution function consists of high- and low-energy components, and
the low-energy one intermittently shows energy dispersion in the
directions parallel and anti-parallel to the local magnetic field.
Considering the time-of-flight effect of the energy-dispersed low-energy
ions, we infer that these ions originate in the region down the tail,
and move along the magnetic field toward the ionosphere and then come
back to the magnetotail by the mirror reflection. The pitch-angle
dispersion analysis gives consistent results on the travelling time and
path length of the energy-dispersed ions. Based on these observations,
we discuss possible generation mechanisms of the energy-dispersed
structure of the low-energy ions during the northward IMF.